In a typical gas turbine engine, compressor air is discharged into a prediffuser, which is part of a combustion section and serves to convert a portion of dynamic pressure to static pressure. A dump diffuser receives the air at the prediffuser exit and supplies it to and around an aerodynamically-shaped cowl, located ahead of the combustion chamber (combustor), typically separating the air into three branches. One branch is the cowl passage to supply air to fuel nozzles and for dome cooling. The other branches are outer and inner diameter (ID and OD) shroud passages, respectively, where air is introduced into the combustor for cooling and to complete the combustion process. A small portion of each of these shroud's air bypasses the combustor and is used for turbine cooling.
Different combustor designs are shown in the prior art, for instance, in U.S. Pat. No. 4,527,386, also assigned to the assignee of the application. So-called "axial combustors" use a configuration in which the prediffuser and combustor inner and outer liners are generally located symmetrically around the burner axis, resulting in the prediffuser and the cowl passage being approximately axially aligned. This design has been successfully applied in low and moderate temperature rise combustors. But, in high temperature rise combustors a greater portion of the airflow is introduced through the front of the combustor, i.e., through the cowl, creating flow conditions with reduced air flow for the shrouds. Two interesting results arise from this: First, shroud passages are designed to minimize both shroud pressure loss and weight, and when shroud flow is reduced, shroud height (area) is reduced accordingly, so much so that increased manufacturing tolerances may occasion significant variations in shroud pressure recovery, resulting in significant variations in air flow and distribution. Second, air flow entering the combustor cowl passage--the point closest to the prediffuser--is taken from the center of the prediffuser, where the total (dynamic plus static) pressure is higher than near-prediffuser-wall air which feeds the shrouds, resulting is lower pressure recovery in the shrouds.